1. Field of the Invention
The present invention relates to a process for producing hardened granules having a high bulk density and a high strength which do not adhere to each other in a short time from an ash (comprising a coal ash and used desulfurizing agent) formed by combustion of a fluidized bed comprising a coal as the fuel and limestone as the desulfurizing agent and to a process for producing a ground material having a high CBR value by pulverizing the granules.
2. Prior Art
Recently in many countries including Japan, it is difficult to secure the import of a great amount of petroleum because of a worldwide unstable supply of petroleum since the petroleum crisis. Under these circumstances, the development of a fuel usable as an energy source in place of the petroleum in order to reduce the dependence on the petroleum in the supply and demand of energy is an important problem from a national point of view. Coal energy thus attracts public attention.
Coal as a fuel has been combusted heretofore mainly by a pulverized coal combustion process. However, recently, the fluidized bed combustion process is attracting attention. In the fluidized bed combustion process, the desulfurization is conducted usually in the furnace. Namely, in this process, a coal as the fuel and limestone as the desulfurizing agent are introduced into the furnace and a fluidized bed is formed in a boiler thereof. The fluidized bed combustion process is superior to the conventional pulverized coal combustion process for the following reasons: (1) a small volume of the furnace suffices, and the volume of the boiler may also be small, (2) the kind of the coal fuel is not strictly limited, (3) the combustion can be conducted at a temperature of as low as 750.degree. to 850.degree. C., troubles due to the coagulation of the ash do not occur and the formation of thermal NO.sub.x is only slight, and (4) the overall coefficient of heat transfer on the surface of a heat transfer water tube is high.
In putting the fluidized bed combustion technique to practical use, a problem of treatment of the ash is posed. The ash formed by the combustion of the fluidized bed comprises a limestone and the used desulfurizing agent. The used desulfurizing agent comprises II-type anhydrous calcium sulfate and unreacted quick lime. The amount of the limestone used is usually determined so that the molar ratio of Ca/S will be 3 to 6 in order to obtain a high efficiency of removing sulfur oxides from the coal combustion gas, i.e. a high desulfurization rate. By the reaction with the sulfur oxides conducted at 750.degree. to 850.degree. C., the limestone is converted into quick lime and II-type anhydrous calcium sulfate, which are discharged together with the coal ash. The amount of the ash formed by the combustion of the fluidized bed varies depending on the kind of the coal, desulfurization rate, operation conditions of the boiler, etc. The amounts of the coal ash, II-type anhydrous calcium sulfate and quick lime are about 15 to 20 wt %, 1 to 10 wt % and 1 to 10 wt %, respectively, based on the coal used.
The ash formed by the combustion of the fluidized bed is disposed of or utilized by the following processes:
(1) a moistening process wherein 10 to 40 parts by weight of water is added at once to 100 parts by weight of the ash to moisten the ash, the mixture is kneaded and then disposed of,
(2) a moistening/granulation process wherein 20 to 50 parts by weight of water is added at once to 100 parts by weight of the ash and the mixture is kneaded to obtain granules to be disposed of, and
(3) a moistening/granulation/steaming process wherein 20 to 50 parts of water is added in two portions to 100 parts by weight of the ash, the mixture is kneaded to obtain granules and the granules are treated with steam at 60.degree. to 100.degree. C. under atmospheric pressure.
However, the moistening process (1) has a drawback in that since the granules have a diameter of 0.01 to 100 mm and they are not sufficiently compacted, their bulk density is low, the quantity thereof carried in one load by a transportation means such as a truck or cargo boat is small and after it is used for the land reclamation, the density and strength of the reclaimed ground are low.
Although dense granules having a diameter of 0.5 to 50 mm can be obtained by the moistening/granulation process (2), this process has the following drawbacks:
(a) the granules adhere to each other because of the surface water thereof and, therefore, they cannot be handled easily,
(b) the density of the product is not far higher than that of the kneaded product obtained in the moistening process, since granules having a diameter of less than 0.5 mm are scarcely contained therein,
(c) the hydration reaction velocity is low, since it is conducted at ambient temperature, and the strength of the solid thus formed is not sufficiently increased and, as a result, the bearing capacity of the ground obtained by the reclamation with the solid is low, and
(d) when the quick lime content is increased to 7.5 wt % or more, the granules are broken by a heat generated by slaking of the quick lime, since water is added at once before the kneading.
The moistening/granulation/steaming process (3) has the following disadvantages:
(a) the granules adhere to each other because of the surface water thereof and, therefore, they cannot be handled easily,
(b) the density of the product is not far higher than that of the kneaded product obtained in the moistening process, since granules having a diameter of less than 0.5 mm are scarcely contained therein, and
(c) the production cost is high, since the atmospheric pressure steam having a temperature of 60 to 100.degree. C. is used for increasing the strength of the granules.
When the quick lime content of the ash is high, a heat is generated by the reaction thereof with water to frequently disintegrate the granules.